Structural adhesives are typically thermosetting resin compositions that may be used to replace or augment conventional joining techniques such as screws, bolts, nails, staples, rivets and metal fusion processes (e.g., welding, brazing and soldering). Structural adhesives are used in a variety of applications that include general-use industrial applications, as well as high-performance applications in the automotive and aerospace industries. To be suitable as structural adhesives, the adhesives should exhibit high mechanical strength and high impact resistance.
Since adhesion is a surface physico-chemical phenomenon, it follows that the physical properties of an adhesive bond depend strongly on the interaction of the structural adhesive with the surface of adherends used to form an adhesive bond. Under ideal conditions, the structural adhesive is applied to the clean surface of an adherend. However, due to cost and processing limitations, the surface of an adherend (e.g., an automotive part) is often contaminated with hydrocarbon-containing materials which, if left untreated, can lead to undesirable bond failure at the adhesive/adherend interface. Contaminants may include mill and corrosion protection oil on steel and aluminum, fingerprints, and other grime and soil found in manufacturing processes and warehousing.
Removing hydrocarbon-containing material from surfaces of adherends can be difficult. Mechanical processes such as dry wiping and/or the use of pressurized air tend to leave a thin layer of the hydrocarbon-containing material on the metal surface. Liquid cleaning compositions can be effective but may be less desirable from a processing point of view because the cleaning liquid must be collected and recycled or discarded. In addition, a drying period is usually required after the cleaning step. Therefore, a need exists within the industry for structural adhesives that form strong adhesive bonds on clean surfaces, as well as surfaces contaminated with hydrocarbon-containing material.